In order to enhance contrast of low absorbing specimen, compared to conventional amplitude contrast images, for example, differential phase-contrast imaging is applied. In WO 2004/071298 A1, an apparatus is provided for generating phase-contrast X-ray images, in which, along an optical path, an incoherent X-ray source, a source grating, a phase grating, an analyzer grating and an image detector are provided. Currently, the fringe pattern generated by the phase grating, which is also referred to as grating G1, at the location of the analyzer grating, which is also referred to as grating G2, is probed by the phase stepping technique with an analyzer grating absorbing typically 50% of the radiation that has already passed the object under study. This latter fact is particularly problematic in medical imaging applications, where the largest possible fraction of transmitted radiation through the patient should be detected and contribute to the image signal in the detector.